Herpes simplex virus (HSV) is part of the larger herpes virus family, including Varicella-Zoster virus (VZV), Epstein-Barr virus (EBV) and Cytomegalovirus (CMV). It is an enveloped double-stranded DNA virus causing infections in humans. HSV is classified into various types, including HSV-1 and HSV-2. The complete genomes of human HSV-1 and HSV-2 have been sequenced (see, e.g., NCBI Accession Nos. NC_001806.1/GI:9629378 and NC_001798.1/GI:9629267, respectively; see also accession numbers X14112 and Z86099, respectively). Both HSV-1 and HSV-2 can cause disease in humans and exposure or infection is fairly common in adult populations. Up to 80% of the U.S. adult population has been exposed to HSV-1 and approximately 20% of the U.S. population has contracted HSV-2 infections.
HSV infection symptoms include the common cold sore found near the lips and also genital herpes. The virus can also cause keratoconjunctivitis, with the potential to lead to blindness, and encephalitis. Once subsided, the virus remains in a latent state inside nerve cells (ganglia) that supply nerve fibres to the infected area. The virus can become reactivated and travels through the nerve fibres back to the skin, thereby causing recurrent disease.
HSV-2 is commonly associated with newborn encephalitis where it is associated with maternal genital infections. HSV-related encephalitis has the highest fatality rate of all types of encephalitis with an annual incidence of 1 to 4 per million. HSV encephalitis affects people of all ages and at any time of the year. In adults, HSV-related encephalitis is thought to be due to a reactivation of a latent virus. Symptoms may include fever, headaches, seizures, an altered level of consciousness and personality changes. The similarity of these symptoms to other maladies makes clinical diagnosis difficult. If left untreated, the mortality rate for Herpes Simplex Encephalitis (HSE) is as high as seventy percent, compared with as low as nineteen percent among those who receive treatment. Of the treated patients, about one third can return to normal function.
One mechanism for transmission of HSV is by sexual transmission. This route of transmission presents a serious consequence of HSV infection in the transmission of the HIV virus. HIV transmission is five times more likely to occur from an HIV/HSV-2-coinfected person with genital ulceration and HIV acquisition is twice as likely in someone sero-positive for HSV-2.
Accurate diagnosis of HSV infection is essential if transmission rates of HSV and its consequences are to be reduced. Although it is not possible to eradicate HSVs from an infected individual, episodic treatment with nucleoside analogue drugs will shorten the duration of the clinical episode and can also reduce the risk of transmission of the virus when continuously administered as daily suppressive therapy. Clinical diagnosis of HSV infection has been reported to have a poor sensitivity of only approximately 40% (Expert Rev. Mol. Diagn. 4, 485-493 (2004); Sex. Trans. Dis. 17, 90-94 (1990)) so rapid reliable tests with good sensitivity and specificity are needed to improve diagnostic accuracy in those with and without symptoms. Tests are also required that differentiate between HSV-1 and -2.
Current diagnostic methods for HSV include viral culture, serological tests and nucleic acid amplification testing (NAAT).
Culture and typing were once considered the gold standard for diagnosis but its usefulness is severely limited by the stage of clinical disease. When testing early vesicular lesions, the culture detection rate is about 90% whereas in older crusted lesions this falls to only 27% (Genitourin. Med. 64, 103-106 (1988)). Another problem with this method is that it is slow since it takes 3 days for the majority of culture isolates to appear positive. The liability of the virus also means that samples must be transported rapidly with maintenance of the cold chain otherwise much reduced sensitivity will result due to, for example, bacterial outgrowth.
Detection of HSV infections has improved dramatically with the advent of type-specific HSV antibody serology testing (Am. J. Clin. Pathol. 120, 829-844 (2003). These tests are sensitive and can distinguish between HSV-1 and HSV-2 antibodies. However, type specific antibody tests suffer from false positive results and are also considered inadequate due to a delay of between two and three weeks in appearance of antibody response after initial infection. The performance of the same test can also vary, giving different sensitivities and specificities depending on the population tested (Clin. Microbiol. Infect. 10, 530-536 (2004)). For these reasons, they are not considered suitable for general population screening.
NAAT testing for HSV provides for the direct detection of viral DNA from specimens by amplifying DNA sequences using HSV-1 or -2 specific primers and has been shown to be superior to culture (Sex. Trans. Infect. 78, 21-25 (2002); Sex. Trans. Infect. 80, 406-410 (2004)) and highly specific as compared to cell culture (J. Infect. Dis. 1345-1351(2003)). Different HSV genes have been identified as targets for DNA amplification, among them, DNA polymerase glycoprotein. NAAT based testing for HSV has utilised Strand-displacement amplification (SDA), PCR, real time PCR and the TaqMan® PCR detection system. NAAT based assays for HSV are now considered to be the gold standard. However, PCR-based amplification assays are not without their limitations. For example, tests may take up to 2 days to complete and require specialized thermo-cycling equipment.
Sciortino et al. (2001) J. Virol. 75, 17 pp. 8105-8116 describe a method for the detection of HSV using reverse transcribed RNAs that were detected by PCR. A set of 90 primers were designed to amplify all of the 84 expressed ORFs of HSV. One primer pair was designed to amplify a portion of the UL42 ORF of HSV-1, hybridising to regions 301 to 322 and 680 to 701 of GenBank Accession No: GU734771.1, GI:290766003, region 92815 . . . 94534. However, the method described therein suffers from the problems associated with PCR-based amplification methodologies and also requires a reverse transcription step which adds yet further complexity to the method. It is also believed that this assay would not be able to discriminate between HSV-1 and HSV-2 nucleic acids.
A need remains for a diagnostic test that provides sensitive and specific detection of HSV in a relatively short time so that infected individuals may be treated promptly to limit morbidity and prevent the spread of infection. A test of this kind that distinguishes between HSV-1 and/or HSV-2 would also be desirable and so a type determination of HSV that is present in the sample can be made.